The transit agency claims the express line's high daily ridership proves it succeeds, but the auditor points out that the express line also runs more trains per day than any other line, so the raw daily total may simply reflect heavier service rather than heavier use. This is an Evaluate question, so the right choice is the one whose answer would most change how strongly the auditor's point holds up.
(A) asks whether the express line carries more passengers per train than the agency's other lines do. This isolates use from service level: if each train on the express line carries more riders on average, that supports the idea that the line is genuinely well used; if not, the daily total is just a byproduct of running more trains. Either answer swings the argument, which is exactly what an Evaluate question needs.
(B) asks about the express line's daily total again, just measured against other cities instead of against the agency's own lines. That keeps the comparison on the same daily-total axis the auditor already flagged as uninformative, since the train-frequency confound is never removed. (C) asks about operating cost, which speaks to efficiency of spending, not to how heavily the line is used relative to its service level. (D) asks whether ridership has grown over time, a separate question from whether the current daily total reflects heavy use or heavy service. (E) asks about rider comfort, an attitude measure that does not test the passengers-per-train relationship the auditor's point turns on.
Only (A) discriminates use from frequency, so only (A) can move the argument's strength in either direction.